Stress balanced extendible boom structure

ABSTRACT

A lazy tong or scissor-type of extendible boom structure having one end mounted on a platform. The boom structure can be extended, elevated, and/or rotated independently by drive structure mounted on the platform and a load-carrying basket is mounted on the opposite end of the boom. The boom comprises a pair of parallel lazy tongs, each comprising pairs of beams which are pivoted to one another in scissorlike fashion, the pivoted beams opposite each other on the two lazy tongs forming one &#39;&#39;&#39;&#39;bay.&#39;&#39;&#39;&#39; Stress transmitting rigid structural cross members are fixed between the inner beams of each bay and similar cross members are nested at interference-free positions and between the outer beams of each bay to balance the torque within the pair of lazy tongs and prevent rotation of the beams and consequent twisting of the extended boom when an asymmetrical load is applied thereto, such as by an off-center load in the platform or basket or by the locating of the platform base in a nonhorizontal plane.

United States Patent 1191 McCollum 1451 Apr. 15, 1975 STRESS BALANCEDEXTENDIBLE BOOM STRUCTURE [75] Inventor: Fenwick Harris McCollum, Los

Angeles, Calif.

[73] Assignee: Del Mar Engineering Laboratories,

Los Angeles, Calif.

22 Filed: July 13, 1973 211 App]. No.: 379,097

Related US. Application Data [63] Continuation of Ser. No. 189,361, Oct.14, 1971.

Primary ExaminerReinaldo P. Machado Attorney, Agent, or FirmGeorge FredSmyth [57] ABSTRACT A lazy tong or scissor-type of extendible boomstructure having one end mounted on a platform. The boom structure canbe extended, elevated, and/or rotated independently by drive structuremounted on the platform and a load-carrying basket is mounted on theopposite end of the boom. The boom comprises a pair of parallel lazytongs, each comprising pairs of beams which are pivoted to one anotherin scissorlike fashion, the pivoted beams opposite each other on the twolazy tongs forming one bay. Stress transmitting rigid structural crossmembers are fixed between the inner beams of each bay and similar crossmembers are nested at interference-free positions and between the outerbeams of each bay to balance the torque within the pair of lazy tongsand prevent rotation of the beams and consequent twisting of theextended boom when an asymmetrical load is applied thereto, such as byan off-center load in the platform or basket or by the locating of theplatform base in a non-horizontal plane.

11 Claims, 6 Drawing Figures STRESS BALANCED EXTENDIBLE BOOM STRUCTUREThis is a continuation of application Ser. No. 189,361, filed Oct. 14,1971.

BACKGROUND OF THE INVENTION The present invention relates to anextendible boom structure which can be utilized to position materialsand/or personnel at desired distant positions relative to a base member.

If desired, the base member can be mounted on a truck or other movabledevice and the boom can be used for such things as repairing streetlamps, building and maintaining electric and telephone systems, treesurgery, etc.

In the past, such extendible booms have usually comprised telescopingtubular systems or extendible boom structures such as those taught inUS. Pat. No. 3,470,981 or in copending application Ser. No. 57,471,filed July 23, 1970 and assigned to the assignee hereof. Such extendiblebooms, arranged with suitable power and control systems and mounted onmobile bases have long been used for platform lifts, vehicle hoists,aircraft cargo leaders, etc., and often are formed, as illustrated inthose references, utilizing parallel lazy tong assemblies which areinterconnected by bars to ensure that the tongs move in unison.

In the usual terminology employed in the extendible boom art, a baycomprises two pairs of beams, one on each lazy tong, each pair beingpivotally connected at the centers of the beams to form two parallelscissorlike structures. In the normal sense, the two structures arepositioned at opposite sides of the boom in such a way that the innerbeams of the structures are parallel to one another and the outer beamsare also parallel to one another. In practice, the two pivot points maybe joined by a rod extending therebetween which aids in holding the twostructures together. Additional bays are added to the boom by pivotallyattaching the bottoms of the beams of each such structure to the tops ofthe beams of the structure immediately below it. Since each structureacts like a scissors, when the bottoms of the beams of the lowest bayare forced toward one another, the tops of the beams are also forcedtoward one another. This action is transmitted through each of the bays,causing the boom to be elongated or extended in the direction in whichthe boom is turned and elevated. Thus, each side of the boom forms alazy tong which cooperates with the lazy tong on the opposite side ofthe boom.

The prior art extendible booms have proven to be reasonably satisfactorywhen used to produce a straight vertical lift of equipment or personnelfrom ground level to a working height above ground level. When the boomis extended at an angle relative to horizontal, the results achievedhave also been substantially satisfactory. However, experience hastaught that if the load in the platform or basket is not centered so asto be equally distributed through the lazy tongs on each side of theboom, or if the truck-bed or base member is parked in a non-horizontalposition, certain side loadings or forces acting against the boom willtend to collapse one of the lazy tongs while tending to extend theopposite tong. The results is at least dangerous and possibly disastrousfor any personnel on the outer end of the boom; the boom tends to twistand may collapse since the twisted boom is incapable of withstanding theforces exerted thereon by the uplifted weight.

In the past, in order to ensure that the two tongs moved together. andin an attempt to eliminate boom twist, the inner beams in each bay whereinterconnected by torsion bars fixed to the inner beams at or adjacentto both ends of the beamsv However, it has been found that such torsionbars are inadequate to prevent the tendency for one of the tongs toextend and the other to collapse and for the outer beams to becomemisaligned when a relatively large asymmetrical load is applied at theouter end of an extended boom.

In fact, it has been determined that the torsion bars fixed between theinner beams of each bay actually aid in the creation of an even moreundesirable condition, in addition to still allowing a small amount ofsuch twist. The imposition of an asymmetrical load which tends tocollapse the lazy tong assembly on the compression side and extend theassembly on the tension side of the boom is resisted by the bars whichproduce torsional moments in each bay, causing the entire boom structureto tend to rotate about its longitudinal axis. Nhen there are only a fewbays, this problem is not very serious since the degree of rotationimparted to the boom is rather small. However, with an increase in thenumber of bays, the rotation of the boom about its axis increasesrapidly, thereby limiting the extended length of the boom. In its worstcondition, during extension ofa boom of this type having a very largelength and an asymmetrical loading condition, the personnel bucket atthe end of the boom could rotate about the boom axis, with an obviouslydisastrous result.

Since such lazy tong or scissor-type booms are relatively easy toassemble and maintain, it is highly desirable to produce such a boomwhich is capable of withstanding such offset loading without allowingthe boom to become twisted and unstable.

SUMMARY OF THE INVENTION The present invention relates to structurewhich may be utilized with a lazy tong-type extendible boom to preventthe boom from becoming twisted or rotated about its axis in the event ofan asymmetrical loading being imposed upon the boom arms. Morespecifically, the present invention relates to structure which may beutilized with such booms so as to equally distribute any asymmetricalloading between the boom arms or lazy tongs, in such a way as to createequal and opposite moments within the bays, thereby preventing them fromattempting to collapse or extend. This may be accomplished byeliminating the induced twisting moments which cause instability andpossible failure of the boom.

In one preferred embodiment of the invention, the inner and outer beamsof each bay are interconnected by torsion bars which are fixed tocorresponding beams on the opposite side of the bay.

In a preferred embodiment, the apparatus comprises torsion barsconnecting the outer beams in each bay of the two lazy tongs assembliesand similar torsion bars connecting the inner beams. The torsion barsfixed to the outer beams are provided with a torsional spring ratesubstantially equal to that of the torsion bars connecting the innerbeams in each bay. Whether or not the bars are rigid, it is importantthat their spring rates be substantially identical. Thus, the inventionis applicable to rigid and non-rigid torsion members.

As a result, the twisting moments in each bay created by the outer beamtorsion bars are equal and opposite to those produced by the inner beamtorsion bars, thus eliminating unbalanced twisting moments and rotationof the boom.

As an additional advantage of this invention, it has been found that itnow becomes possible to use a single boom-extending power system,located midway between the two lazy tong structures, to extend andretract the boom. Since the extendible boom is thus made more stable,the structure allows the achievement of a system in which elevation,extension, and rotation of the boom about the base can be independentlycontrolled, allowing precise positioning of the platform or basket atthe end of the boom.

Booms manufactured in accordance with the present invention cantherefore be utilized to move personnel and equipment much greaterdistances from the base or platform upon which the boom is mounted. Ofeven greater importance is the fact that the personnel and/or equipmenton the platform or basket of the boom are provided with a degree ofsafety heretofore unobtainable with prior art devices since theimposition of asymmetrical loads will not cause twisting or rotation ofthe boom which could result in failure and/or injury.

As an additional advantage of the invention, it will be realized uponreading the following description that the torsion bars on the inner andouter beams in successive bays will serve as convenient equally spacedladder rungs if a workman should otherwise be stranded in the bucket ofan extended boom during an equipment malfunction.

Further objects, advantages, modes, and embodiments of the presentinvention will readily be understood by those skilled in the art byreference to the following Detailed Description and accompanyingdrawings which illustrate what is presently considered to be onepreferred embodiment of the best mode contemplated for utilizing thenovel principles of the invention as defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective illustrationof an extendible boom formed in accordance with the present invention,illustrating the boom mounted on a truck and extended relative thereto;

FIG. 2 is a perspective view of a portion of the boom illustrated inFIG. 1, enlarged so as to show more details of the control structurethereof;

FIG. 3 comprises an isometric illustration of four bays of such anextendible boom in a partially extended position;

FIG. 4 is a sectional illustration of one bay of a boom formed inaccordance with the present invention as seen along a line IVIV in FIG.3;

FIG. 5 is a view similar to FIG. 4, taken along a line V-V of FIG. 3;and

FIG. 6 comprises a side elevation of two bays of a boom such as thatshown in FIG. 1 in the collapsed position.

DETAILED DESCRIPTION As shown in FIG. 1, a vehicle 11 supports a basestructure 13 upon which an extendible boom, generally indicated at 15,is supported. In the particular illustration of FIG. 1, the boom 15comprises two lazy tongs which, together, comprise an exemplary sevenbays,

each of which is made up of two beam structures 17 and 19. Each beamstructure 17 comprises an outer beam 21 and an inner beam 23 while eachbeam structure 19 comprises an outer beam 25 and an inner beam 27. Innerbeams 23 and 27 are parallel to one another and outer beams 21 and 25are also parallel to one another. Thus, each pair of opposed beamstructures 17 and 19 form one bay and, when a plurality of bays arecombined as shown in FIG. 1, a lazy tong is formed on each side of theboom as clearly illustrated.

At the outer end of the boom, a basket or platform 29 is suspended fromthe ends of a pair of parallel end beams 31 which extend from and aresupported by the seventh or last bay. When the boom is in use, thebasket 29 is suspended from the beams 31 in an upright position underthe influence of gravity. If desired, a control panel 33 may be mountedon the basket to allow a workman in the basket to control the positionthereof and a similar panel (not shown) may be mounted on the truck 11.

Referring now particularly to FIGS. 1 and 2, it can be seen that thebase structure 13 supports a platform 37 upon which are mounted onelevation control unit 39, an extension unit 41, and a rotation unit 43.Also, if desired, the platform 37 may support a bank of batteries 49which may be utilized to operate the elevation, extension, and rotationcontrol units 39, 41, and 43.

The elevation unit 39 and extension unit 41 may comprise individualjackscrews or hydraulic devices but, whatever type of devices areselected, they may be individually operable, if desired, to provide moreprecise control of the boom. The rotation unit 43 may, for example,comprise an electric motor which drives a pinion gear 45. If the motoris fixed to the platform 37, the gear 45 will act against a ring gear 47fixed on the support 13 to turn the platform and, thus, the boom.However, regardless of the type of structure used to rotate the boom, itmay also be individually operable so as to provide precise boom control.

Elevation control unit 39 is attached at one end to a cross bar 51 whichis fixed between dual perpendicular track assemblies 53 and 55. Thetrack assemblies are pivotally attached to the platform 37 at pivotpoints 59 and 61 so that when the elevation unit 39 is actuated theforce which it exerts against the cross member 51 causes the trackassemblies 53 and 55 to pivot upwardly. For example, as shown in FIG. 1,the track assemblies are in a substantially intermediate position whichis achieved as a result of partial actuation of the elevation system. InFIG. 2, on the other hand, the track assemblies have been raised to thenearly vertical position by the assembly 39. It will be realized thatthe unit 39 also controls the descent of the boom, which can, forexample, be lowered to a substantially horizontal location.

Comparison of FIGS. 1 and 2 will also reveal that, as stated previously,the rotation assembly 43 can be selectively utilized, for example byactuation of a control in the panel 33 or by a similar located at groundlevel, to rotate the support platform 37 to any desired positionso as toproperly orient the boom 15 in that direction.

Referring once again to FIG. 2, it can be seen that the beams on theopposite sides of the first bay are interconnected by a centrallylocated bar 63. The extension unit 41 is attached to the bar 63 at itsmidpoint, and the bar is attached at each of its ends to both the innerbeams 23 and 27 and outer beams 21 and 25, by means of bearings. Sincethe bar 63 is connected to all of the beams in the bay by means ofbearings, when the extension unit 41 pushes against the bar 63, the barwill not have any tendency to rotate relative to unit 41 but will pushthe pivot point on both sides of the bay upwardly. Due to the operationof structure to be described below, this causes the upper ends of theinner and outer beams on both sides of the bay to move toward oneanother in a scissorlike action. Of course, this results in theextension of the boom because the bottom ends of the pivoted sets ofbeams in the second bay are thus caused to be brought together andsimilarly to cause the upper ends of the pivoted beams in the second bayalso to be brought toward one another. A comparison of FIGS. 2 and 6will clearly illustrate that this causes the boom to be extended,achieving a result such as that shown in FIG. 1.

The points of pivotal attachment of the bar 63 to the beams on each sideof the first bay, and thus the points of pivotal attachment of the beamsto one another, and controlled in motion by means of rollers 71 whichextend from the pivot points on each side of the bay. The rollers 71cooperate with the portions of the channels 53 and 55 which extend inthe axial direction of the boom. Thus, the rollers 71 are free to movein the axial direction but are restricted against movement in any otherplane.

On the other hand, the bottom ends of the four beams of the lowest bayare provided with rollers 73 which cooperate with those portions of thechannels 53 and 55 which extend perpendicular to the axial direction ofextension of the boom. In other words, rollers 73 are free to move inline ofdirection which is perpendicular to the extension axis of theboom but are restricted against traveling along any other plane relativethereto. The movement of the various portions of the first bay of theboom, as controlled by the extension unit 41, is strictly limited alongpaths within predetermined planes. Consequently, the movement of everyother bay ofthc boom will also be limited since the motion of each bayis strictly controlled by the motion of the bay immediately below it.

As is clearly illustrated in FIG. 2, the outer beams 21 and 25 of thefirst bay are connected together by means of a torsion bar 75 located atthe lower ends thereof. As shown in FIG. 1, the lower ends of the innerbeams of the first bay are similarly interconnected by a torsion bar 77extending therebetween.

The torsion bars 75 and 77, and other torsion bars described below. arefixed to the opposite beams to ensure the maintenance of the spacingbetween the beams, to ensure equal movement of opposed beams in eachbay, and to properly and equally distribute forces exerted on each bayinto the beams thereof. Near the upper ends of the beams of the firstbay, the outer beams are again connected by a torsion bar 79 and theinner beams are connected by a torsion bar 81. Thus, the torsion bars75, 77, 79, and 81 strengthen and distribute moments within the firstbay.

Referring now to the second bay as shown in FIG. 2, it is seen that apair of torsion bars 83 and 85 connect the inner beams 23 and 27 while asecond set of torsion bars 87 and 89 interconnect the outer beams. Inother words the outer beams in each bay from the second outwardly areinterconnected in the manner shown for the purpose described. Thus eachbar maintains the spacing between the beams of the two tongs, ensuresequal opening and closing of the beams in each tong, and, as describedbelow, distributes moments exerted on the bays in substantially equalamounts.

It will be recalled that in the prior art only the inner beams in eachbay were connected at both the upper and lower ends thereof. whereas theouter beams were not so interconnected. Thus, the present inventionresults in a system in which distribution of forces and moments isprovided so as to prevent rotation of the boom about its axis.

This can be clearly illustrated by reference to FIGS. 35. For example,when side loads A are applied to the outer end of the boom, for example,by asymmetric loading of the platform or bucket 29 or by the force ofgravity acting on the bucket when the truck 11 is parked in a non-levelposition, a reaction force B is generated at the lower end of the boom.Thus, the forces A and B exert a moment on the boom which must beresisted by forces C and D. It will become clear from studying FIG. 3that forces C tend to collapse the lazy tong assembly comprised of beamstructures 17 at the same time that forces D tend to extend the lazytong assembly comprised of beam structures 19. In other words, the innerbeams in each assembly 17 tend to rotate in a counterclockwise directionat the same time that the inner beams in the assembly 19 tend to rotatein a clockwise direction as viewed in the drawings. These rotationaltendencies are illustrated by the arrows E and F shown in FIGS. 4 and 5.

Since the inner beams are interconnected by the members 83 and 85,thereby preventing their rotation in opposite directions, a torsionalmoment G, acting in a clockwise direction, i.e., opposite to thedirection of rotational tendency of the beam, is introduced at eachinner beam of the assembly 17 and an equal and opposite torsional momentH is introduced in the inner beam of each bay within the lazy tongassembly 19. The torsional moment G acting within the lazy tong assembly17 deflects that assembly in the direction indicated by the arrow .1 atthe same time that the torsional moment H acts on the assembly 19 todeflect it in the direction indicated by the arrow K. It can thus beseen, by referring to the direction of the arrows J and K in FIG. 3,that the entire boom structure therefore has a tendency to twist aboutits central axis Z in a counterclockwise direction as viewed from above.

This is the situation as it presently prevails in the prior artstructures. In other words, the torsion bars connecting the inner beamsprevent one lazy tong from extending while the other is retracting but,as a result, causes the entire boom to twist or rotate about its axisand create a dangerous condition which is now believed to be readilyapparent in view of the above force analysis.

Under the loading conditions previously described, the outer members 21of the lazy tongs 17 tend to rotate in a clockwise direction, asindicated by the arrow L in FIG. 5, and the outer beams 25 of the lazytongs l9 tend to rotate in the counterclockwise direction as indicatedby the arrow M in FIG. 4. These rotational tendencies may be resisted bythe torsion bars 87 and 89 which exert a torsional moment N (FIG. 5)acting in a counterclockwise direction on beam 21, and a torsionalmoment P (FIG. 4) acting in the clockwise direction on beam 25.

If torsion bars 83, 85, 87, and 89 are designed so as to have equaltorsional spring rates and so that the sum of the torsional spring rateof any pair of bars connecting inner beams is equal to the sum of thetorsional spring rate of any pair of bars connecting the correspondingpair of outer beams, the twisting of the entire structure will beeliminated. It is well known by those having an elementary knowledge ofdynamics that when two bars or tubes of the same material, for example,are of different lengths, their spring characteristics differ merely aresult of the difference in length. In order to ensure that the shortertorsion bars 83 and 85 have the same spring rate characteristics as thelonger bars 87 and 89 then, the latter two bars need be made onlyslightly larger in diameter than bars 83 and 85. Of course, if tubes areused, this same result can be achieved by controlling the diameterand/or the wall thickness thereof. This difference in diameter is, formost materials, insufficient to be discerned in the scale in which thedrawings have been made. The torsion spring rates might also be equatedby the selection of different materials for the different length torsionbars. In other words, with equal spring rates, the torsional momentsintroduced by bars 83 and 85 will be equal and opposite to the torsionalmoments introduced by bars 87 and 89. Consequently, the twisting momentson the entire boom structure will be balanced and any tendency of thestructure to rotate about its axis will be eliminated. In other wordsthe rotational moment illus' trated by the arrow Z in FIG. 3 will beexactly offset by a rotational moment which is equal to that illustratedby the arrow but extends in the opposite direction.

As a further advantage of the invention, the use of the torsion barsconnecting the inner and the outer beams of each bay allow thesatisfactory use of the single extension unit 41 to extend both lazytongs of the boom, since the torsion bars 83, 85, 87, and 89 allow thetongs to resist the forces C and D, thus keeping equal forces at eachend of the bar 63 balanced about its midpoint where the extension unit41 is attached. This, of course, reduces the weight and cost of theentire unit. In other words, this invention provides structure forresisting the twisting action of a boom under asymmetrical loading,including rigid members connecting the outer beams of the two lazy tongassemblies, the rigid members having comparable rigidity to similarmembers connecting the inner beams in the assemblies. As a result, equaland opposite twisting moments are created in each bay under conditionsof asymmetrical loading, thereby eliminating unbalanced twisting momentswithin each bay.

As an added benefit of the use of this invention. it will be realizedthat if there should be any equipment malfunction while the boomstructure is substantially ex tended, a person riding in the bucket willbe able to climb down the boom, using the torsion bars in each bay asrungs of a ladder, thereby substantially increasing his safety.

Thus, the applicant has provided an embodiment of a new and improvedconcept in the art of extendible booms, which concept yields a trueadvancement in that art due to the increase in safety and elimination ofthe possibility of structural failure. Many modifications andalternations of the above-described embodiment will now become apparentto those skilled in the art, but such modifications, alternations,additional embodiments, etc, will not, merely as a result of anydiffcrences with the above-described embodiment, exceed the true scopeof the invention as defined in the following claims.

Therefore, I claim:

1. A boom for locating a basket suspended from the end thereof at apredetermined position in which an asymmetrical load may be applied tosaid boom comprising a pair of lazy tongs arranged in parallelrelationship,

each including a plurality of bays comprising an inner beam,

an outer beam,

means near the centers of said inner and outer beams for pivotallyconnecting them together,

means interconnecting said pair of lazy tongs to prohibit rotation ofsaid boom about the central axis thereof when an asymmetrical load isapplied thereto by said basket, which load tends to extend one of saidlazy tongs and retract the other comprising first torsion transmittingmeans fixed between the inner beams in each bay intermediate and pivotconnection means and each end thereof and second torsion transmittingmeans fixed between the outer beams in each bay intermediate said pivotconnection means and each end thereof, said first and second torsiontransmitting means being so constructed as to have equal torsionalspring rates, and

means at the outermost bay for supporting and mounting a basket thereon.

2. The boom of claim 1 including means acting only along the centralaxis of said boom for selective extension and retraction thereofregardless of whether said boom is subject to uniformly distributed orasymmetrical loads.

3. The boom of claim 1 including mounting means to which said boom isattached for support thereof, and

independently actuatable means for elevating, rotating, and extendingsaid boom.

4. A boom comprising a pair of lazy tongs forming a plurality ofpivotally connected bays, each bay including a pair of parallel innerbeams, one in each lazy tong,

a pair of parallel outer beams, one in each lazy tong, and

means pivotally connecting the inner beam and the outer beam of eachlazy tong in each bay near the centers of said inner and outer beams,means for suspending support apparatus from the outermost bay of theboom, whereby asymmetrical loading may be imposed on the boom, therebycausing one lazy tong to tend to retract and the other to extend,

a pair of first torsion transmitting means extending between the innerbeams in each bay and fixed thereto to prevent rotation of said innerbeams upon the application of asymmetrical loading forces to saidsupport means, and

a pair of second torsion transmitting means extending between the outerbeams in each bay and fixed thereto to prevent rotation of said lazytongs about the central axis of said boom upon the application ofasymmetrical loading forces to said support means, said first and secondmeans being so constructed as to have identical torsional spring rates.

5. The boom of claim 3 including a support means,

guide means mounted on said support means, and

means on the lowermost beam for cooperating with said guide means tocontrol the extension and retraction of said boom.

6. In a boom comprising a pair of parallel lazy tongs, each including aplurality of bays comprising an inner and outer beam, which support awork platform in suspension therefrom at the outer ends thereof andincluding apparatus for preventing the collapse of one lazy tongs andthe extension of the other as a result of asymmetrical loading beingapplied to said boom,

means for preventing the rotation of said boom about the central axisthereof as a result of torsion forces generated by said collapse andextension means comprising at least one torsion transmission memberextending between and fixed to the outer beams in each bay of said boom,and

at least one torsion transmission member extending between and fixed tothe inner beams in each bay of said boom, the torsional spring rate ofsaid lazy tong extension and collapse prevention means in any given baybeing equal to the torsional spring rate of said boom rotationprevention means in the same bay 7. The boom of claim 6 including asupport means upon which the inner ends of the boom is mounted, and

means on said support means for exerting forces along the central axisof said boom to uniformly extend and retract the latter regardless ofasymmetry of uniformity of loading of said boom by said platform.

8. The boom of claim 7 wherein said force exerting means comprises asingle drive means and means, situated generally along the central axisof said boom, connecting said single drive means to a cross member, and

a cross member in the lowermost bay of said boom pivotally attached tothe lazy tong on each side of the boom for transmitting extension andretraction forces from said connecting means to said lazy tongs.

9. The device of claim 6 including means mounted on said support meansfor independently altering the rotational location of said boom relativeto said support means.

10. The device of claim 6 including means mounted on said support meansfor independently altering the angle of elevation of said boom relativeto said support means.

11. In a boom comprising a pair of parallel lazy tong, each including aplurality of bays comprising an inner and outer beam, from which a loadbearing member is suspended at the outer end of the boom. which loadbearing member may subject one lazy tong to collapsing forces and theother lazy tong to extension forces as a result of asymmetrical forceloading, the method of prohibiting such non-uniform movement of the lazytongs while also prohibiting the rotation of the outer end of the boomrelative to the inner end thereof about the central axis of the boomcomprising the steps of transmitting torsional forces exerted on theinner beam of each lazy tong in each bay to the opposite inner beam inthe other lazy tong in the same bay via members having a first lengthand a first total spring rate, and

transmitting torsional forces exerted on the outer beam of each lazytong in each bay to the opposite outer beam in the other lazy tong inthe same bay via members having a second, longer length but said firsttotal spring rate.

1. A boom for locating a basket suspended from the end thereof at apredetermined position in which an asymmetrical load may be applied tosaid boom comprising a pair of lazy tongs arranged in parallelrelationship, each including a plurality of bays comprising an innerbeam, an outer beam, means near the centers of said inner and outerbeams for pivotally connecting them together, means interconnecting saidpair of lazy tongs to prohibit rotation of said boom about the centralaxis thereof when an asymmetrical load is applied thereto by saidbasket, which load tends to extend one of said lazy tongs and retractthe other comprising first torsion transmitting means fixed between theinner beams in each bay intermediate and pivot connection means and eachend thereof and second torsion transmitting means fixed between theouter beams in each bay intermediate said pivot connection means andeach end thereof, said first and second torsion transmitting means beingso constructed as to have equal torsional spring rates, and means at theoutermost bay for supporting and mounting a basket thereon.
 2. The boomof claim 1 including means acting only along the central axis of saidboom for selective extension and retraction thereof regardless ofwhether said boom is subject to uniformly distributed or asymmetricalloads.
 3. The boom of claim 1 including mounting means to which saidboom is attached for support thereof, and independently actuatable meansfor elevating, rotating, and extending said boom.
 4. A boom comprising apair of lazy tongs forming a plurality of pivotally connected bays, eachbay including a pair of parallel inner beams, one in each lazy tong, apair of parallel outer beams, one in each lazy tong, and means pivotallyconnecting the inner beam and the outer beam of each lazy tong in eachbay near the centers of said inner and outer beams, means for suspendingsupport apparatus from the outermost bay of the boom, wherebyasymmetrical loading may be imposed on the boom, thereby causing onelazy tong to tend to retract and the other to extend, a pair of firsttorsion transmitting means extending between the inner beams in each bayand fixed thereto to preVent rotation of said inner beams upon theapplication of asymmetrical loading forces to said support means, and apair of second torsion transmitting means extending between the outerbeams in each bay and fixed thereto to prevent rotation of said lazytongs about the central axis of said boom upon the application ofasymmetrical loading forces to said support means, said first and secondmeans being so constructed as to have identical torsional spring rates.5. The boom of claim 3 including a support means, guide means mounted onsaid support means, and means on the lowermost beam for cooperating withsaid guide means to control the extension and retraction of said boom.6. In a boom comprising a pair of parallel lazy tongs, each includes aplurality of bays comprising an inner and outer beam, which support awork platform in suspension therefrom at the outer ends thereof andincluding apparatus for preventing the collapse of one lazy tongs andthe extension of the other as a result of asymmetrical loading beingapplied to said boom, means for preventing the rotation of said boomabout the central axis thereof as a result of torsion forces generatedby said collapse and extension means comprising at least one torsiontransmission member extending between and fixed to the outer beams ineach bay of said boom, and at least one torsion transmission memberextending between and fixed to the inner beams in each bay of said boom,the torsional spring rate of said lazy tong extension and collapseprevention means in any given bay being equal to the torsional springrate of said boom rotation prevention means in the same bay.
 7. The boomof claim 6 including a support means upon which the inner ends of theboom is mounted, and means on said support means for exerting forcesalong the central axis of said boom to uniformly extend and retract thelatter regardless of asymmetry of uniformity of loading of said boom bysaid platform.
 8. The boom of claim 7 wherein said force exerting meanscomprises a single drive means and means, situated generally along thecentral axis of said boom, connecting said single drive means to a crossmember, and a cross member in the lowermost bay of said boom pivotallyattached to the lazy tong on each side of the boom for transmittingextension and retraction forces from said connecting means to said lazytongs.
 9. The device of claim 6 including means mounted on said supportmeans for independently altering the rotational location of said boomrelative to said support means.
 10. The device of claim 6 includingmeans mounted on said support means for independently altering the angleof elevation of said boom relative to said support means.
 11. In a boomcomprising a pair of parallel lazy tong, each including a plurality ofbays comprising an inner and outer beam, from which a load bearingmember is suspended at the outer end of the boom, which load bearingmember may subject one lazy tong to collapsing forces and the other lazytong to extension forces as a result of asymmetrical force loading, themethod of prohibiting such non-uniform movement of the lazy tongs whilealso prohibiting the rotation of the outer end of the boom relative tothe inner end thereof about the central axis of the boom comprising thesteps of transmitting torsional forces exerted on the inner beam of eachlazy tong in each bay to the opposite inner beam in the other lazy tongin the same bay via members having a first length and a first totalspring rate, and transmitting torsional forces exerted on the outer beamof each lazy tong in each bay to the opposite outer beam in the otherlazy tong in the same bay via members having a second, longer length butsaid first total spring rate.